Delphinidin

Bonds are forces that form molecules by attracting and holding atoms together. Bonds can form in several different ways. The number of electrons in an atom’s outermost energy level determines how many chemical bonds can be formed by that particular atom. If the number of electrons in the outermost energy level is less than eight, the atom may lose, gain, or share electrons, resulting in an outermost energy level that contains the maximum number of electrons. Three types of chemical bonds are of particular significance for living organisms:

1. Covalent Bonds
Form when two atoms complete their outermost energy level by sharing a pair of electrons in the outermost orbital; they hold two or more atomic nuclei together and travel between them, keeping them at a stable distance from each other. For example, the single orbital of a hydrogen atom, which has just one electron, is usually filled by attracting an electron from another hydrogen atom. As a result, two hydrogen atoms share their single electrons, making a combined orbital with two electrons. The combined orbital, with its two hydrogen atoms, forms a molecule of hydrogen gas. The covalent bond is shown as a single line, so that hydrogen gas (H2) is depicted as H—H. Except for hydrogen and helium, which have only one orbital, elements can have up to four more orbitals in each energy level. Carbon atoms, for example, have six electrons—two in the innermost orbital and one in each of the four outer orbitals of the second shell; by covalent bonding, carbon can share four electrons. When four hydrogen atoms bond to one carbon atom, a molecule of methane gas (CH4) is formed.

When one pair of electrons is shared, the bond is said to be single. When two pairs of electrons are shared, the bond is referred to as double, and triple bonds are formed when three pairs of electrons are shared. Double bonds are shown in structural formulas with double lines (e.g., C=C), and triple bonds are shown with three lines (e.g., C=N). In covalent bonds involving molecules such as those of hydrogen (H2), where electrons are shared equally, the bonds are said to be nonpolar. However, polar covalent bonds (e.g., those of a water molecule) are formed when electrons are closer to one atom than to another and therefore are shared unequally. Because the electrons are shared unequally, parts of the molecule are not electrically neutral and are slightly charged. Covalent bonds are the strongest of the three types of bonds discussed here and are the principal force binding together atoms that make up some important biological molecules.

2. Ionic Bonds
In nature, some electrons in the outermost orbital are not really shared but instead are completely removed from one atom and transferred to another, particularly between elements that can strongly attract or easily give up an electron. Molecules that lose or gain electrons become positively or negatively charged particles called ions. Ionic bonds form whenever one or more electrons are donated to another atom and result whenever two oppositely charged ions come in contact. Ions are shown with their charges as superscripts. For example, table salt (sodium chloride) is formed by ionic bonding between an ion of sodium (Na+) and an ion of chlorine (Cl–). The sodium becomes a positively charged ion when it loses one of its electrons, which is gained by an atom of chlorine. This extra electron makes the chlorine ion negatively charged, and the sodium ion and chlorine ion become bonded together by the force of the opposite charge.

Some ions, such as those of magnesium (Mg++), give up two electrons and therefore have two positive charges. Such ions can form ionic bonds with two single negatively charged ions such as those of chlorine (Cl–), forming magnesium chloride (MgCl2). Many biologically important molecules exist as ions in living matter.

3. Hydrogen Bonds
Form as a result of attraction between positively charged hydrogen atoms in polar molecules and negatively charged atoms in other polar molecules. Negatively charged oxygen and/or nitrogen atoms of one molecule may attract positively but weakly charged hydrogen atoms of other molecules, forming a weak bond. Hydrogen bonds are very important in nature
because of their abundance in many biologically significant molecules. They have, however, only about 7% to 10% of the strength of covalent bonds. Hydrogen bonds help cellular processes by maintaining the shapes of proteins such as enzymes, which make different compounds fit together precisely to complete a chemical reaction.